The field of the disclosure relates generally to a drive circuit for an electric motor, specifically an induction motor and, more specifically, a drive circuit that enables synchronized transfer to drive circuit operation.
At least some known induction motors are fixed speed motors that operate most efficiently at line frequency power. Such induction motors exhibit uncontrolled acceleration during startup. Further, at low load conditions, such induction motors operate less efficiently. Alternatively, an induction motor may be driven with a variable speed motor controller to adapt motor speed to a load level. Such configurations are generally limited by power factor, electromagnetic interference, and electrical losses.
Providing a drive circuit for induction motors enables efficient operation at both high and low load conditions. For example, an induction motor operating a compressor in a heating, ventilation and air conditioning (HVAC) system may experience high load conditions during peak temperatures and low load conditions during milder temperatures. The drive circuit operates the induction motor using an inverter in a low-speed mode under low load conditions, and operates the induction motor using line frequency power in a high-speed mode under high load conditions.
Where two speed operation of an air moving motor and/or a fluid moving motor is sufficient for an application, a drive circuit may be used that has a reduced power rating relative to a power rating of the induction motor. Using a lower-power drive circuit improves efficiency at low speeds; however, if the drive circuit is undersized compared to the motor rating, it is unable to deliver the full current or full torque necessary to drive the induction motor at full speed.
The lower relative power rating of the drive circuit presents an issue when transitioning from line frequency power operation of the induction motor to drive circuit operation. When the line frequency power is removed, the induction motor is spinning at full speed and at 60 Hertz (Hz) line frequency, and a frequency of the drive circuit must be synchronized to the frequency of the spinning induction motor. If the drive circuit attempts to synchronize with the motor frequency while the motor is spinning too quickly, the inverter switches may become overwhelmed by the full torque and full power being regenerated back to the drive circuit, and an over-current condition may occur and damage the inverter switches. Furthermore, energy may be regenerated back into the DC bus capacitor of the drive circuit and may cause an over-voltage condition and damage the DC bus capacitor.